(i) Field of the Invention
The present invention relates to a method of rapid consolidation and drying of a continuous moist porous web and, more particularly, to a method of rapidly consolidating and drying a moist paper web.
(ii) Description of the Prior Art
Techniques presently employed in the paper industry tend to treat pressing and drying as two separate operations--mechanical removal of some water, together with consolidation of the web taking place in the presses, followed by heat application in the dryer section to remove the remaining water thermally to achieve the desired dryness.
In recent years, improvements in wet pressing have been achieved by utilizing improved clothing, i.e. press felts, multinip presses, increased dwell-time in the nip (e.g. the extended nip press) and by preheating the web (e.g. steam boxes, infra-red radiation). However, despite the improvements there are few commercial operations achieving a post-press dryness in excess of 50% solids. Drying is typically completed by passing the web over a series of rotating cast-iron cylinders which are heated internally with steam. Drying rates achieved by this method are low, necessitating a multiplicity of cylinders to achieve the required dryness of the web. Hence, a large capital investment is required initially and a high ongoing cost is incurred in maintaining the complete drying section in good working order (including syphons, steam traps, pumps, valves, fabrics, ventilation and heat recovery equipment etc.)
There have been proposals in the art, as exemplified by Wahren in U.S. Pat. No. 4,324,613, to greatly improve the rate and efficiency of drying a paper web, thus overcoming some of the disadvantages of the presently used methods. In this type of system, heat transfer to the pressing surface (in the above case a rotatable roll) is via a gaseous or liquid medium which is less than 100% efficient. In the case of a gaseous heat transfer medium, a heat recovery system has to be incorporated to reduce heat loss. In the case of a liquid heat transfer medium, a recirculating system has to be incorporated and, with it, attendant sealing problems. In both cases, the overall heating systems become more complicated and expensive. The alternative of heating by means of electric resistance elements embedded in the roll surface is also complicated because electric power must be fed through brushes or slip rings into the rotating roll.
In U.S. Pat. No. 3,702,912, Greenberger describes a method and apparatus for calendering strip-like material using induction heating to heat the roll surfaces through the material being processed. Larive (U.S. Pat. No. 4,384,514 and Cdn patent No. 1,143,039) describes the use of multiple induction coils to control the nip profile of (for example) a calender by selective operation of coils to locally heat, and therefore increase the diameter of the roll. These patents do not address the high heat generation and transfer rates required for drying as taught herein.
However, heating a substantially ferromagnetic surface such as a roll by means of alternating current induction coils provides distinct advantages over the methods taught by Wahren in that:
1. The heat is generated within and very close to the surface of the roll and heating is therefore achieved more efficiently than heat transfer to the roll from hot gases or a liquid medium and
2. The induction coils may be simply mounted in close proximity to the roll surface and there is no need for the complicated and costly construction of heat recovery systems or the seals that would be necessary in the case of heating via a liquid medium, or of brushes or slip rings which would be required by roll-mounted electric resistance elements.
Generally, it has been accepted by the art that relatively high temperatures are desirable when utilizing drying technologies such as taught by Wahren. This can, however, in turn lead to problems with the material forming the porous surface and also with respect to the metallurgy of the heated surface.